Current detector and current measuring apparatus including such detector with temperature compensation

ABSTRACT

A current sensing device and a residual current detection device are described having a temperature compensation capability so that residual current can be directly measured to a high degree of precision in the background of a high load current. The residual current device comprises a plurality of resistive shunts ( 14 ) for connection in respective ones of a plurality of lines through which current can flow to and from a load, and detector means ( 15 ) sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing through the shunts. The temperature compensation means ( 15   h ) is provided for facilitating compensation for fluctuations in shunt resistance with variations in temperature. The current sensor comprises a rigid metallic link member having two end portions ( 13 ) of conductive material and an intermediate portion ( 14 ) interconnecting the end portions. The intermediate portion is formed of a resistive material, and an integrated circuit analog to digital converter ( 15 ) is mounted on said link member. The converter has analog input terminals connected to respective ones of said two end portions and digital output terminals for connection to a processing apparatus. A temperature sensor ( 15   h ) is provided on or within said intermediate portion.

[0001] This invention relates to a current sensor with a temperaturecompensation capability intended for use in an electrical apparatus suchas a residual current detection device.

[0002] JP 04083175 discloses a current detector formed by joining aconductor to a circuit substrate on which a voltage measuring circuit ismounted.

[0003] U.S. Pat. No. 6,028,426 discloses a current measuring apparatusfor measuring current in a shunt which includes a temperature sensor forsensing temperature of the shunt.

[0004] It is an aim of the present invention to provide a current sensorcapable of directly sensing current or voltage supplied to a load and aresidual current detector in economical form which includes temperaturesensing means for facilitating temperature compensation.

[0005] In accordance with the invention there is provided a currentsensor comprising a metallic link member having two end portions ofconductive material and an intermediate portion interconnecting the endportions, said intermediate portion being formed of a resistivematerial, and an integrated circuit analog to digital converter mountedon said link member, said converter having analog input terminalsconnected to respective ones of said two end portions and digital outputterminals for connection to a processing apparatus, wherein atemperature sensor is provided in the integrated circuit mounted ontosaid intermediate portion.

[0006] The temperature sensor is preferably an electronic semi-conductortemperature sensor and may be mounted directly onto the intermediateportion with a thermally suitable or compatible conducting glue. Thetemperature sensor may be built into (i.e. integrated) into theintegrated circuit analog to digital converter in which case it willform part of a semi-conductor die mounted directly onto the intermediateportion.

[0007] Embodiments of the invention have the advantage that thetemperature sensor will follow the temperature of the intermediateportion very closely. It is therefore possible to compensate for changesin the resistance of the intermediate portion resulting from temperaturevariations as the current flowing therethrough changes.

[0008] Conveniently, the converter is attached to the intermediateportion by a layer of electrically insulative adhesive material and theanalog input terminals of the converter are connected to the endportions by wire bonds.

[0009] The converter preferably includes a delta-sigma modulator whichprovides a high frequency one-bit digital data. One or more decimationfiltering stages may be included in the converter.

[0010] The converter may also have a voltage reference terminal forconnection to a reference voltage source, the converter operating toprovide digital output signals respectively representing the currentflowing through said intermediate portion and digital output signalsrepresenting the voltage on one of said end portions.

[0011] Embodiments of the invention may be advantageously employed inresidual current devices. Conventionally, residual current is detectedutilising a current transformer having primary windings through which,in the case of a single phase device, load current flows in oppositedirections so that if the return current is different from the outwardlyflowing current because of current leakage an output current signal isinduced in a secondary winding of the transformer. In the case of amulti-phase device, primary windings of the transformer are connected inall of the phase lines and the neutral line. In normal situations, whenthere is no current leakage, the net current induced in the secondarywinding is zero and therefore no output is detected.

[0012] Sophisticated materials have been developed for the core of thecurrent transformer, which enable considerable accuracy to be obtainedwhen the currents flowing in the primary windings are substantiallysinusoidal. However, switch mode power supplies are often used forcomputers and other equipment and there is an increasing tendency forsuch equipment to cause dc offsets in the currents. Such developmentshave made detectors utilising current transformers less reliable andprone to false tripping or failure to detect a dc current leakage.

[0013] This is a particular problem in the case of directly actuatedelectromechanical devices, where the current transformer secondarywinding actually drives an actuator. The situation is not much improved,when including an electronic detection and amplification means connectedto the secondary winding, as there are still problems with highfrequency transients and dc offsets. A very small dc current level cancause the core to saturate thereby seriously impairing the ability ofthe detector to detect current leakage.

[0014] It is also an aim of the present invention to provide a residualcurrent detection device in which the above mentioned problems aresubstantially overcome in a simple and efficacious manner.

[0015] In accordance with the invention there is further provided aresidual current detection device comprising a plurality of resistiveshunts for connection in respective ones of a plurality of lines throughwhich current can flow to and from a load, and detector means sensitiveto the voltage developed across each of the shunts to detect anyimbalance between the currents flowing through the shunts, the detectormeans comprising a converter in the form of an integrated circuitmounted on each of the resistive shunts and temperature compensationmeans is provided for facilitating compensation for fluctuations inshunt resistance with variations in temperature.

[0016] In preferred embodiments, the temperature compensation means is atemperature sensor provided on or within each of said plurality ofresistive shunts.

[0017] Preferably, the detector means comprises an analog to digitalconverter for each shunt and a processor for receiving the digitalsignals from the converters and determining whether a current imbalanceexists. In this case, the temperature sensor may be built into (i.e.integrated) into the analog to digital converter. The temperature sensoris preferably an electronic semi-conductor temperature sensor mounteddirectly onto the shunt with a thermally conducting glue.

[0018] Each shunt preferably takes the form of a composite strip havingconductive portions at its ends and a resistive portion interconnectingthe conductive portions. Such composite strips can be mass producedinexpensively to very high tolerances which makes them extremelysuitable for this purpose.

[0019] The analog to digital converter for each shunt may include adelta-sigma modulator, which generates a high frequency single-bit datastream which is converted by decimation filtering to a multi-bit digitalstream at a lower frequency.

[0020] The analog to digital converter for each shunt is preferablyconnected to the processor through an isolation barrier so that theconverter can float at the voltage level of the shunt which it serves.The decimation filtering may be effected entirely in the converter,entirely in the processor or split between the converter and theprocessor.

[0021] Embodiments of the invention provide for a high degree ofaccuracy in the measurement of the current and voltage in the circuitbeing monitored. It is desirable to measure the temperature of thecurrent sensor for the purpose of compensating for changes in ambienttemperature.

[0022] In order to detect residual currents of 1 to 10 mA in thebackground of a power supply current in the order of 100's of Amps, thedegree of precision must be in the order of one part in 100,000. Auseful residual current device is operative to detect residual currentsin the order of a few to tens of mA, typically such that the circuit istripped when the residual current detected reaches about 30 mA. However,less sensitive residual current tripping thresholds can be set such asto trip at residual current levels as high as 100 mA. This is a degreeof precision several orders of magnitude greater than the degree ofprecision required for power metering applications and the like where itis sufficient to measure current to an accuracy of 1%. It is thereforeadvantageous to provide temperature compensation in accordance withembodiments of the invention in order to ensure accurate measurement ofthe residual current. In a preferred embodiment, the temperature of thecurrent sensor is measured to an accuracy of plus or minus 0.25° C. toplus or minus 0.5° C. and the device may be calibrated over the expectedtemperature range, i.e. minus 5° C. to 85° C.

[0023] Voltage measurement is required for power metering but apractical embodiment might measure to an accuracy of 1% although in apreferred embodiment, the voltage is measured to an accuracy of one partin about 1000 or better.

[0024] As each of the current, voltage and temperature are measuredindependently, it is possible to sense other functionalities such as oneor more of power metering, current metering and arc fault protection.For example, abnormally high current measurements for extended periodsmay result from overloading, short circuits or line-to-ground faults andsuch events. Other events may also be detected, such as abnormalvariations in current profile with respect to time. Such events may beindicative of faults which would not activate a residual current device.A wide variety of electrical characteristics are exhibited by at leastsome arcing faults and by monitoring the current and voltage with theprecision afforded by embodiments of the invention, it is possible todetect situations where arc faults may be occurring. This monitoring mayinclude detecting voltage drops indicative of arcing events or comparingthe usual peak voltage of the circuit to the actual circuit voltage inthe timed vicinity of the usual occurrence of the peak voltage. Forexample, peak voltages occur at 90 degree phase angles from the zerocrossing point of the voltage. Voltage associated with the occurrence ofan arcing fault is significantly reduced in the vicinity of the 90degree phase angle.

[0025] The problem is that the energy levels of many arc short circuitsis insufficient to trip many, if not all, conventional circuit breakersand some conventional fuses. By employing the current and voltagedetection in embodiments of the invention, the current and voltagecharacteristic of the circuit can be compared against prescribedcriteria which represent arc fault conditions. The characteristics ofarc faults are discussed in various publications of UnderwritersLaboratories, Inc. (UL) including: “Technology For Detecting AndMonitoring Conditions That Could Cause Electrical Wiring System Fires:Contract No: CPSC-C-94-1112, September 1995”; “The UL Standard ForSafety For Arc-Fault Circuit Interrupters, UL1699, First Edition, datedFeb. 26, 1999”.

[0026] The invention will now be further described by way ofnon-limiting example with reference to the accompanying drawings, inwhich:

[0027]FIG. 1 is a diagrammatic perspective view of an example of theinvention as applied to a single phase device,

[0028]FIG. 2 is a block diagram of another example of the invention asapplied to a three phase device,

[0029]FIG. 3 is a perspective view showing one of the current sensingdevices embodying the present invention,

[0030]FIG. 4 is a sectional view of the current sensing device of FIG.3,

[0031]FIG. 5 is an elevation of the device of FIG. 3,

[0032]FIG. 6 is a block diagram of a simple form of the electroniccircuit of a single current sensor device,

[0033]FIG. 7 is a block diagram of an alternative form of the electroniccircuit,

[0034]FIG. 8 is a block diagram of yet another form of the electroniccircuit, and

[0035]FIG. 9 is a block diagram of a form of the electronic circuitwhich incorporates a temperature sensor in accordance with an embodimentof the present invention.

[0036] In the device shown in FIG. 1, a substrate 10 supports twocomposite conductor strips 11, 12. Each of these includes end portions13 of copper and an intermediate portion 14 of a resistive material suchas manganin. The strips are formed by slicing up a sandwich formed byelectron beam welding the copper portions to opposite sides of themanganin portion. The shunts formed by the resistive portionsmanufactured by this method can have a nominal resistance of 0.2 mg to atolerance of less than 5%. If the two shunts 14 used on one device arepressed from adjacent portions of the sandwich stock, they are matchedto within 2%. Differences between characteristics of any two devices arepredominantly linear. Hence, calibration of the shunts built into a unitat two different temperatures can virtually eliminate shunt errors. Inthis way, at least two temperature measurements are made. Twotemperature measurements are taken because the difference in shunt Afrom shunt B is linear when the devices are adjacent to one another.

[0037] However, it is desirable to provide for direct compensation fortemperature fluctuations arising from current fluctuations especially ina single current detector. The resistivity of an ideal precisionresistance material is not changing with temperature. Compared to puremetals such as copper or aluminium with Temperature Coefficient ofResistance (TCR) values close to 4000 ppm/DegC, the TCR values ofManganin or Zeranin are more than a factor of 400 better over thetemperature of interest—but still not zero.

[0038] In reality, the plot of Resistance Vs Temperature (R(T)−curve) isnot strictly linear and it is common practice to describe the curves bya third order polynomial. In general this is:

[0039] R(T)=R_(o)* (1+a_(o)*T+b_(o)*T²+c_(o)*T³) where T=Temperature inDegC and R₀=Resistance at 0 DegC.

[0040] At a more practical Reference Temperature of 20 DegC we canrewrite this as R(T)=R_(20*[) 1+a_(o)*(T−20)+b_(o)*(T−20)²+c_(o)*T(−20)³]

[0041] The typical curves for the resistance materials Manganin andZeranin curves are determined by the main composition of the alloys andvary very little from batch to batch. The production spread is less than5 to 10 ppm/DegC. These slight differences in the TCR value can beexpressed in a tiny change of the first order coefficient “a_(o)” or“a_(2o)” in the above equation and the second and third coefficients arebasically not changed. For example a dR(T)/R20−curve for differentbatches is just rotated around the 20 DegC point and the curve itself isunchanged. This explains the calibration of the shunts at two differenttemperatures mentioned above.

[0042] However in accordance with the present invention, it may bedesired to calibrate each individual shunt in an RCD or in the case ofthe Current Sensor (Single Shunt) it could be calibrated separately.Varying as a third order polynomial at least 4 if not more points for agood calibration would be needed.

[0043] As described by the above equations if the temperature and ourRef resistance R₂₀ are known for example it is possible, with a suitablenumber of points, to find the coefficients and calibrate the shunt.

[0044] In the preferred embodiment, the temperature sensor itself(indicated generally by reference numeral 15 h in FIGS. 4 and 9) ifbuilt into (integrated) and is a part of the ASIC which includes theanalogue to digital converter ADC. In other words the temperature sensorwill be an electronic semi-conductor temperature sensor in the ADC. TheADC is mounted as a semi-conductor die directly onto the shunts with asuitable thermally conducting glue and will therefore track thetemperature of the Manganin (shunt) very accurately.

[0045] Preferably, the temperature sensor output is sampled via thevoltage modulator. It could have its own modulator (see RCD FIG. 9showing the added temperature channel) or be multiplexed into thevoltage channel (see FIG. 2).

[0046] For calibration it is possible to avoid having to make severalstable temperatures to make the measurements. Instead a measurement canbe made at 20 DegC for example and then a known current applied whichheats up the shunts. Several measurements can be made during thisprocess until the shunt arrives at its new steady state temperature as aresult of the applied current.

[0047] In the example shown in FIG. 1, there is a separate signalpre-processing ASIC 15 mounted on each of the shunts 14 and connected tothe copper end portions 13 of the associated conductor strips. The twoASICs 15 are connected to via an isolation transformer array 16 to amain processor 17. The ASICs 15 operate to convert the two voltagesacross the shunts into a digital signal stream which is communicated tothe processor 17 via the isolation transformer array. The main processoris programmed to provide a drive signal to a trip actuator 18.

[0048] The actual preferred structural configuration of the currentsensors is shown in FIGS. 3 to 5. These show leads 40 connecting twoanalog input terminals of the ASIC to the two copper end portions 13.Other leads connect other terminals of the ASIC 15 to a lead frame 40 aby means of which all other external connections are made. FIG. 5 showsin dotted lines a block 42 on encapsulation material and FIG. 4 shows anelectrically insulative adhesive layer 41 by means of which the ASIC isattached to the intermediate portion 14, which may be of manganin orzeranin of the composite strip 14, 15. The strips are formed by slicingup a sandwich formed by electron beam welding of the copper bars toopposite sides of a manganin bar. The temperature sensor is preferablyintegrated within the ADC of the ASIC 15.

[0049]FIG. 6 shows that within the ASIC 15 there is provided a singledelta-sigma modulator 15a. There is also an analog input circuit whichhas its input terminals connected to the copper end portions 13. Theoutput of the ASIC 15 in this case consists of a high frequency one-bitdata signal train. In use, the ASIC output is connected via atransformer or other isolation barrier 16 to a processor 17. Theprocessor in this arrangement is configured to carry out one or moredecimation filtering operations to convert the one-bit signal streaminto a multi-bit value at a lower frequency.

[0050] The processor 17 may typically be configured to receive signalsfrom a plurality of the detectors and to sum these signals to ascertainwhether the current flows through the detectors are balanced. Such anarrangement can be used for residual current detection allowing anactuator to trip a switch if an unbalanced condition is found to exist.The processor 17 may alternatively or additionally compare theinstantaneous current level with a trip level so that overcurrenttripping can be controlled.

[0051]FIG. 2 shows in rather more electrical detail a three phasedevice. In this case there are four shunts 14, one in each phase lineand a fourth in the neutral line. The ASICs 15 of FIG. 1 are shown asfour separate blocks 20, 21, 22 and 23, and there is a power supply unit24 which draws power from the phase lines on the mains side of theshunts 14 and provides controlled voltages to the processor 17. Power issupplied to the four blocks 20 to 23 via isolation barriers 25 whichmake up the array 16. Each block of the ASIC includes an analog todigital converter in the form of a delta-sigma modulator which providesa high frequency one-bit digital data stream. A multiplexer may beincluded in each converter so that the converter can provide to theprocessor, through the respective isolation barrier, signalsrepresenting both current in the associated shunt and the voltage at oneend of it. The processor, uses these signals to monitor the current ineach shunt and to operate the actuator 18 if an imbalance occurs.

[0052] It will be noted that the voltage sensing connections to theASICs are made via resistor chains connected between each phase line andthe neutral. Each such resistor chain comprises an outer pair ofprecision resistors of relatively low ohmic value and an intermediateresistor of relatively high ohmic value.

[0053] These resistor chains allow the RCD to be provided with anindependent reference. If the neutral ADC is taken as the selectedsystem reference, then the operating software of the main processor canuse the multiple signals derived from the several resistor chains tocalibrate each phase against the neutral reference.

[0054] The CPU is programmed to carry out the necessary calculations todetermine the existent of an imbalance and can determine the true RMSvalue of the residual current, which conventional devices fail to docorrectly particularly in the case of non-sinusoidal current waveforms.The CPU may be programmed to enable it to determine from the data itreceives whether a particular event is, in fact, an unacceptable leakagemore reliably than conventional devices. For example, the CPU can takeinto account the historic performance of the unit when setting theleakage current threshold and may ignore events which have arecognisable “signature”. In this way improved tolerance to nuisancetripping can be obtained.

[0055] Decimation filtering of the high frequency one bit data stream isrequired to reduce each data stream to a multi-bit digital signal at apredetermined sample frequency. By way of example, each current signalmay be a 23-bit signal at a sample rate of 64 times the mains frequency,but lower resolution at lower sample rates can be employed whennon-linear, rather than linear conversion is acceptable. The decimationfiltering is typically a function of the processor, filtering of thefour data streams being executed simultaneously so that sample valuesare derived for all four shunts simultaneously. A circuit employing suchan arrangement is shown in FIG. 6 as described above.

[0056] In an alternative embodiment as shown in FIG. 7, one or morestages of the decimation filtration may be executed by hardware includedwithin the ASIC. This includes a serial output driver 15 b to transmitthe bits of the multi-bit digital signal produced by the filtrationstage 15 c serially to the processor. Multi-bit digital words aretransmitted serially across the isolation barriers instead of a one-bitsignal stream. The filtration stages may be split between the ASIC andthe processor. With this arrangement, the configuration of the processorcan be simplified as part or all of the decimation filtration operationis carried out in the ASIC.

[0057] Where the current and voltage are both to be monitored as in thesystem shown in FIG. 2, the circuit 15 may be as shown in FIG. 8 withseparate modulations and filtering components for the two signal streamsand a common serial interface. Alternatively separate serial interfacesmay be employed. The ASIC of FIG. 8 has a further analog input which canbe connected to a reference voltage. source. Two analog input stages arepresent and these feed signals to two independent delta-sigma modulators15 d, 15 e. As shown, there are two independent decimation filtrationstages 15 f, 15 g for the two one-bit digital signal streams. Theoutputs of the stages 15 f, 15 g may, as shown be connected to a commonserial output stage or (not shown) separate serial output stages may beprovided.

[0058] It will be appreciated that the arrangement of FIG. 8 may bemodified by the omission of the two filtration stages 15 f, 15 g whereall filtration is to be carried out by the processor.

[0059] Where voltage as well as current is monitored by the processor,precise calibration of the shunts can be achieved. This allows moreaccurate determination of the current balance in RCD applications.Moreover, as voltage and current are both being monitored to a highlevel of precision, accurate power consumption metering can be obtained.

[0060] Where the devices of the invention are used in RCD andovercurrent trip systems, the processor can be programmed to recognisethe transients which may occur when loads are switched in and out ofcircuit to avoid false tripping. Many other convenient functions can beprogrammed into the processor, made possible by the high precision ofthe current measurements capable of being carried out.

[0061]FIG. 9 shows an arrangement similar to the one of FIG. 8 exceptfor the addition of a temperature sensor 15 h in accordance withembodiments of the present invention. The temperature sensor 15 h isinput and sampled via the voltage modulator. The sensor could have itsown modulator or be multiplexed into the voltage channel (FIG. 9) asmentioned above. The arrangement of FIG. 9 is operative to combine in aserial data stream the input parameters of current I, voltage V andtemperature T.

[0062] The arrangements described enable very accurate detection ofcurrent imbalance to be effected even in the presence of switchingtransients and DC offsets. The problems which arise from potentialsaturation of the current transformer core are avoided completely.

[0063] Since the CPU receives actual line current and voltage data fromeach of the blocks 20 to 23, it can be programmed to perform othercalculations, such as current limit and power consumption. Thus an RCDdevice constructed as described above can also provide the functions ofa conventional circuit breaker and/or those of a power consumption meterwithout any additional sensing or analog-to-digital components beingrequired. The device may also be adapted to perform arc faultprotection.

1. A residual current detection device comprising a plurality ofresistive shunts for connection in respective ones of a plurality oflines through which current can flow to and from a load, and detectormeans sensitive to the voltage developed across each of the shunts todetect any imbalance between the currents flowing through the shunts,the detector means comprising a converter in the form of an integratedcircuit mounted on each of the resistive shunts and [wherein a]temperature compensation means [is provided] for facilitatingcompensation for fluctuations in shunt resistance with variations intemperature.
 2. A device as claimed in claim 1, in which each converter[the detector means] comprises an analog to digital converter for eachshunt and the detector means includes a processor for receiving thedigital signals from the converters and determining whether a currentimbalance exists.
 3. A device as claimed in claim 1 or claim 2, in whicheach shunt takes the form of a composite strip having conductiveportions at its ends and a resist portion interconnecting the conductiveportions.
 4. A device as claimed in claim 2 or claim 3, in which theanalog to digital converter for each shunt includes a delta-sigmamodulator which produces a high frequency single bit digital streamwhich is converted by decimation filtering into a multi-bit digital datasteam at a lower frequency. cm
 5. [6.] A device as claimed in anypreceding claim [5], in which each integrated circuit has analog inputterminals connected by lead wires to the two copper end portions of thecorresponding one of the resistive shunts.
 6. [7.] A device as claimedin claim 5 [6], in which the integrated circuit also has a terminalconnected to a voltage reference source and includes a second converterfor providing a digital stream dependent on the voltage on one of thecopper end portions of the associated one of the shunts.
 7. A device asclaimed in any preceding claim wherein the temperature compensationmeans comprises a temperature sensor in each said integrated circuit. 8.A current sensor comprising a rigid metallic link member having two endportions of conductive material and an intermediate portioninterconnecting the end portions, said intermediate portion being formedof a resistive material, and an integrated circuit analog to digitalconverter mounted on said link member, said converter having analoginput terminals connected to respective ones of said two end portionsand digital output terminals for connection to a processing apparatus,wherein a temperature sensor is provided [on or within] in theintegrated circuit mounted onto said intermediate portion.
 9. A currentsensor as claimed in claim 8 in which the converter is attached to thelink member by means of a layer electrically insulating adhesive.
 10. Acurrent sensor as claimed in claim 9 in which the converter is attachedto the intermediate portion.
 11. A current sensor as claimed in claim 10in which the analog input terminals of the converter are connected tothe end portions by means of wire bonds.
 12. A current sensor as claimedin any one of claims 8 to 11 in which the converter has a voltagereference terminal for connection to a reference voltage source and saidconverter operates to provide digital output signals representing thecurrent through said intermediate portion and digital output signalsrepresenting the voltage on one of the end portions.
 13. A currentsensor as claimed in any one of claims 8 to 11 in which said converterincludes a delta-sigma modulator which provides a high frequency one-bitdigital data stream.
 14. A current sensor as claimed in claim 13 inwhich the converter also includes at least one decimation filter stage.15. A current measurement apparatus including at least one currentsensor as claimed in any one of claims 8 to 14 and a processor circuitconnected to receive and process digital signals received from saidcurrent sensor.
 16. A current measurement apparatus as claimed in claim15 in which the processor circuit is configured to carry out one or moredecimation filtering operations on the received digital signals.
 17. Acurrent measurement apparatus according to any one of claims 8 to 16 ora residual current detection device according to claim [any one ofclaims 1 to] 7, wherein the temperature sensor is an electronicsemi-conductor temperature sensor mounted directly onto the intermediateportion or shunt with a thermally suitable conducting glue.
 18. Acurrent measurement apparatus or a residual current detection deviceaccording to claim 17, wherein the temperature sensor is integrated intothe integrated circuit analog to digital converter.
 19. A currentmeasurement apparatus or a residual current detection device accordingto any one of the preceding claims further comprising one or more ofpower metering, circuit breaking and arc fault protection.